1. Field of the Invention
The present invention relates to lock-up capacity control apparatus and method for a torque converter in which a lock-up capacity of the torque converter is appropriately controlled when an engine is switched to a no-load state during an acceleration slip lock up in which the lock-up capacity of the torque converter is augmented by means of a time series control in accordance with a load state of the engine.
2. Description of the Related Art
Since a torque converter performs a power transmission under a torque augmentation and under a torque variation absorption along with a relative revolution (a slip) between input and output elements of the torque converter, a smooth power transmission can be achieved but a transmission efficiency is reduced. Therefore, many of lock-up type torque converters have been used in each of which the relative revolution (the slip) between the input and output elements of the torque converter is limitable during a low-load, high-speed power transmission in which the torque augmentation function and the torque variation absorption function are not required. Then, in order to enhance the transmission efficiency of the torque converter and to improve a fuel consumption of an engine which constitutes a vehicular power train together with the torque converter and an automatic transmission, it is effective to expand a lock-up vehicle speed region in which a direct coupling (lock up) between the input and output elements of the torque converter is carried out to limit the relative revolution (slip) between the input and output elements to a low vehicle speed at all possible, along with an extension of a fuel supply cut-off time due to a reduction suppression of an engine revolution at a time of no-load of the engine. Especially, in a case where the automatic transmission is a continuously variable transmission such as a toroidal continuously variable transmission or V-belt continuously variable transmission, it is possible to remarkably reduce a lock-up release vehicle speed to a lower vehicle speed. The improvement of the fuel consumption due to the expansion of the lock-up vehicle speed region becomes remarkable. On the other hand, the reduction of the lock-up release vehicle speed to the low vehicle speed raises the following problems. That is to say, when a drive wheel has a tendency of lock due to an abrupt braking of the vehicle, a driving stop of the engine (an engine stalling) occurs unless a quick release of the lock up of the torque converter is carried out since the drive wheel having the tendency of lock is directly coupled with the engine. In a case where the lock-up release vehicle speed is set to be low, the torque converter is in a lock-up state up to the low vehicle speed. The drive wheel becomes the tendency of lock and it becomes difficult to release the lock up of the torque converter before the engine stalling occurs and it becomes difficult to avoid a problem of the engine stalling when the abrupt braking occurs.
A Japanese Patent Application First Publication No. 2002-106707 published on Apr. 10, 2002 exemplifies a previously proposed lock-up control apparatus in which a coast lock-up capacity control in which, during a coast running with an accelerator pedal released (engine no-load state) carried out before a vehicle braking, a lock-up capacity of the torque converter is, although the lock-up capacity is different according to a coast running torque, reduced to a capacity immediately before the torque converter slips by means of a learning control is proposed. According to this coast lock-up capacity control technique, since the coast lock-up capacity is small, a lock-up release response of the torque converter can accordingly be higher. Even if the drive wheel becomes the lock tendency due to the abrupt braking of the vehicle, a quick lock-up release of the torque converter can avoid an occurrence of the engine stalling. However, in the coast lock-up capacity control technique, a transmission torque becomes large when a re-acceleration with the accelerator pedal depressed is carried out so that the slip occurs in the torque converter due to an insufficient lock-up capacity and an engine racing occurs. Therefore, during such a re-acceleration as described above, as described in the above-identified Japanese Patent Application First Publication, an acceleration slip lock up in which the lock-up capacity of the torque converter is gradually augmented (increased) by means of a predetermined time series (ramp) control is proposed.